make sure there's absolutely nothing in the grotto. Sometimes there's mulch there that's hidden (you just have to press A near the spot to check). Once you've cleared the area of items, you can redo the 256 steps to make ditto appear.Hey.
I've been trying over and over what you said and even after at least 10 attempts the Ditto isn't appearing. I've tried starting White 2 a second or so and after the clock reaches 0 but to no avail. How can I fix this? :(
Edit: don't know if onicon mentioned this earlier, but you should set your encounter type to "Hidden Grotto". Currently you have it set to "Wild Pokemon". If you're using a DS original or Lite, you shouldn't have to use EonTimer. You can just wait until the timer gets to one second before the target. Walk the 256 steps WITHIN THE GROTTO. leave and then reenter.